It’s generally accepted among effective altruists that voting in national elections is a valuable thing to do. This post argues that the same logic means that participating in party leadership contests has an even higher expected value. The costs of participating are extremely small, and the odds of having a decisive impact are far higher than for a national election.
Edit: This argument applies across the political spectrum. One of the best arguments for political party participation is similar to voting i.e. getting a say in the handful of leading political figures. We recommend that effective altruists consider this as a reason to join the party they are politically sympathetic towards in expectation of voting in future leadership contests. We're involved in the Labour Party - and Labour currently has a leadership election with only a week left to register to participate. So this post focuses on that as an example, and with a hope that if you're Labour-sympathetic you consider registering to participate. We definitely do not suggest registering to participate if you're not Labour-sympathetic. Don't be a 'hit and run entryist' (Thanks Greg for the comments!).
Example: The Labour leadership election
For example, for just £4.38, you could be one of around 500,000 individuals who could play a decisive role in the upcoming Labour leadership contest.
We believe that, if you're a UK citizen or have lived in the UK for the last year, you should pay £4.38 to register to vote in the current Labour leadership, so you can help decide 1 of the 2 next candidates for Prime Minister. This vote will also impact on which party is in government, and the likelihood of either the Conservatives or Labour being in government.
This is a time limited opportunity for highly leveraged impact. Only people who register by joining the Labour Party by 20 January can vote in the leadership campaign. For £4.38, you have a reasonable chance of determining the next candidate PM, and therefore having an impact in the order of billions of pounds.
We assume you are persuaded by the standard effective altruist view in favour of voting, as argued by e.g. 80,000 Hours or Will MacAskill in Doing Good Better, and based in decision theory and political science literature. Briefly, you have a small chance to be the deciding vote between options with a large difference in impact, so in expected value it is worth your time.
(This is not a thought experiment. In the 2017 Virginia House of Delegates election, a single vote would have flipped the state legislature from Republican to no overall control, leading to a potentially significantly different set of policy decisions .)
The argument is even more persuasive for voting in a leadership election, as the chance of influencing the outcome is much greater and the impact is the same.
In a General Election, you are one vote in tens of millions, e.g. 1 in 32 million at the 2019 General Election. And most people live in safe seats where their vote doesn't make a difference. Even so, it's still worth voting! But your vote is worth even more in a selection. In this selection, you are one vote in hundreds of thousands, e.g. in the 2016 Labour Leadership Election you would have been 1 in ~506,000, and your view is counted equally to everyone else's. Labour uses an alternative vote system where if your candidate is knocked out, your vote goes to your second preference (and so on) - so your vote is even more valuable.
Differences between candidate Prime Ministers are huge. Some are drastically more likely to become PM than others. And different PMs can have drastically different impact.
For an effective altruist, that can be the difference between billions of pounds of effectively spent foreign aid or not. The difference between much more humane animal farming legislation and not. Serious funding and international effort in climate change, nuclear weapons, emerging technological risks (like AI and bio) or not. Even if you’re unsure which of the candidates stand on these issues, you have the three month campaign period to find out more - and who if anyone deserves your vote.
A toy model is imagine you have a 1/500,000 chance of being the deciding vote, and the difference in impact was £1bn. The expected value of a vote would be £2000. This is 10x the cost of membership. Note these assumptions are incredibly conservative.
Joining the party is the easiest way to register to vote in the leadership election  . You can do so by filling in a form on the Labour Party website. The deadline to join as a member if you want to be eligible to vote in the leadership election is 5pm on the 20th of January.
You can always cancel your membership (though of course I'd rather you'd stay a member). The easiest way to do so is just by cancelling your direct debit and waiting until your membership is revoked. See the New Statesman site for more options. Edit: We're certainly not advocating joining just to cancel - we're saying you're not bound in if you change your mind.
Written by a UK group of people involved in the Labour Party and effective altruism. Any questions? If you'd like to discuss Labour and effective altruism, please contact David Lawrence at dc_lawrence[at sign]icloud.com
 Out of 100 seats, The Republicans had already won 50 seats and the Democrats 49 seats. Republican David Yancey and Democrat Shelly Simonds both received 11,608 votes and were forced to go to a tie-breaker, where David Yancey won, leaving the Republicans with 51 seats, a 2-seat majority. A single vote for Shelly Simonds would have led to a 50-50 split in the legislature and therefore power-sharing between the two parties, leading to a potentially significantly different set of policy decisions. See Vox.
 There are other ways to register for a vote. The other options are to become a Registered Supporter, or join a group affiliated to the Labour Party: either a Socialist Society or an affiliated trade union.
Becoming a Registered Supporter will cost £25 (at least £20.5 more than becoming a member). Applications to become a registered supporter will open at 5pm on the 14th of January and only be open for 48 hours, until 5pm on the 16th of January.
You can also join an affiliated group and become an Affiliated Supporter.
One option is to join one of the Socialist Societies which include the Fabian Society, the Jewish Labour Movement, SERA (Labour’s Environmental Campaign) etc. The Fabian Society is likely to be the cheapest short-term option, with a standard membership costing £4.90/month and £2.50/month for under 21s, students, the unwaged and retired.
You can also join one of the trade unions affiliated to the Labour Party. These include the three biggest trade unions in the UK: Unison, Unite, and GMB. The latter two, Unite and GMB, are open to those working in any profession and have types of membership available for the unemployed, students and retired. Membership rates will depend on your circumstances but appear to be ~£14-15/month for those in full-time work.
For both of these options, after you join the Socialist Society or a trade union, you will then need to register as an Affiliated Supporter by 5pm on Monday 3 February, which you can do here.